As master regulators of homeostatic bone remodeling, mature osteocytes embedded in the three-dimensional (3D) lacuna-canalicular network (3D-LCN) structure are known to sense local compressive strain and initiate strain-dependent new bone formation by osteoblasts, utilizing cytokines such as sclerostin and Dkk1 expressed by the osteocytes as major signaling molecules. For example, mouse ulna loading studies elegantly show that higher strain regions of the ulna bone result in less production of sclerostin and increased local osteogenesis at those regions.
Despite these important understandings, a significant challenge remains for in vivo studies of osteocytes due to the difficulty of accessing deeply embedded osteocytes in bone tissues. As yet, there is currently no in vitro model that is capable of reproducing the physiological phenotype and mechanotransduction function of osteocytes for routine use in biomedical research and preclinical drug evaluation. This problem may occur for several reasons: (1) the phenotypic function of primary osteocytes harvested from animal bones cannot be maintained during conventional two-dimensional (2D) culture; (2) commonly used osteocyte-like cell lines such as MLO-Y4 are sufficiently altered so that they do not express sclerostin at detectable levels; and (3) in vitro differentiation of osteoblasts into 3D mature osteocytes with network formation and sclerostin expression has not been realized.
A novel in vitro bone tissue model with a reconstructed 3D osteocyte network could be extremely useful for studying fundamental biological mechanisms associated with osteocytes as master regulators of bone remodeling. The model can be preliminarily validated by reconstructing the bone-like tissue with a 3D mouse osteocyte network and comparing it to the in vivo mouse data. Furthermore, the comparison could provide significant new insights and promote new developments in culturing primary human osteocytes and extending the model's capability for simulating human bone remodeling, including osteocyte-regulated bone formation and bone resorption. Operated at a microfluidic scale, such a human 3D bone tissue model may complement (or possibly replace) animal testing in preclinical evaluation of authentic human tissue response to drugs (e.g., sclerostin antibodies that are being actively pursued for treating the approximately 10 million osteoporosis patients in the U.S., and for treating bone metastases, which presently cause about 350,000 deaths every year in the U.S.